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Humidity in grow tent for seeds cannabis

Humidity in grow tent for seeds cannabis

Over the last decade, more and more states have chosen to legalize the sale of both medicinal and recreational marijuana. Even without national legalization, the U.S. marijuana market was worth $9.2 billion in 2017 and is projected to reach $47.3 billion by 2027, a growth rate of 514% over 10 years (source: Arcview Market Research and BDS Analytics).

Investors are flocking to the marijuana industry, seeing it as one of the safest long-term bets in recent history. Many of them are directing their investments toward dispensaries rather than growers, according to Scott Greiper, president of Viridian Capital Partners.

As the number and size of dispensaries grow, so do consumer expectations of the quality of marijuana products and the availability of diverse and unique brands. As in the wine industry, premier brand names can command a price ten or more times that of an average product.

On behalf of their customers, dispensaries are looking for consistent quality and a range of products. So while the opportunity to run a lucrative and stable growing business has increased dramatically, there is more and more pressure for growers to produce a consistently high-quality product in order to turn increased demand into increased dollars.

Let’s Get Growing

What are the most important points of growing a consistent crop to build your brand? Again, borrowing from the winemaker’s metaphor, it’s all about what you’re growing, where you’re growing it and the conditions it’s grown under. Below, we’ll look at how to create the best growing environment, with a focus on humidity and temperature controls .

To start with, relative humidity is determined by how much water vapor the air can hold at a specific temperature. Warm air can hold more water than cold air, so the warmer the air is, the higher the relative humidity will be .

As you probably already know, after choosing a substrate (the material you’ll be growing your plants in/on), the strain(s) of cannabis you’ll be growing, lights, watering mechanisms and nutrients needed, the most important element of your day-to-day operation will be managing temperature and humidity.

Each stage of a plant’s life-cycle is prone to different needs. With the right equipment, these needs are easily managed when the focus is on keeping everything within the desired ranges and as constant as possible.

To get started, you’ll want to buy a hygrometer and a thermometer. With these tools, you can accurately monitor and then alter humidity and temperature conditions in an attempt to strive for repeatable quality levels time and time again.

Shop Humidifiers and Dehumidifiers

The Four Stages of Plant Cycle

Seedling Stage

  • Seedlings and clones like high humidity levels of 65-70%
  • Temperatures with lights on: 68-77 F° (lights off: 8-9 F° lower)

The increased humidity levels allow seedlings to take water in through the leaves while their root systems are developing.

Vegetation Period

  • Humidity levels can be lowered by 5% each week (acceptable range: 40-70%)
  • Temperatures may be increased slightly if desired.
  • Temperatures with lights on: 71-82 F° (lights off: 8-9 F° lower)

As the roots are now more developed, they’re able to perform most of the needed water uptake with evaporation through the leaves now cooling the plants.

Flowering Period

  • Humidity levels need to be lowered to 40-50%
  • IMPORTANT– Never above 60% humidity!
  • Temperatures with lights on: 68-79 F° (avoid high temperatures)

Late flowering (1-2 weeks before harvest)

While not strictly a necessity, the following can have a direct impact on yield, flavor and appearance:

  • Bring down humidity levels as much as you can: 30-40%
  • Lower daytime temperatures, and also increase the temperature difference (day/night)
  • Temperatures with lights on: 64-75 F° (lights off: minus 9-18 F°)

Adjusting Humidity Levels and Temperature

Depending on your lighting solution, season a specific crop is grown in (assuming year-around production), the outdoor air vented in (as it will vary in temperature and its own relative humidity), controlling temperature and humidity in your growing space will be one of the tougher challenges you will face on a day-to-day basis.

While it’s tough, just know that every grower working at a scale similar to you is going through this as well.

Ways to Lower Humidity

  • Run your existing tube vent on higher velocities or upgrade your air flow fans. This applies to both air exits in tents and the room it is placed in! Increase fresh/cool air supply.
  • Try to water your plants right after lights are switched on. The room will be cooler at that time and the immediate absorption will result in less overall humidity.
  • The most consistent option will be: Invest in a decent sized dehumidifier–small humidifiers don’t have large enough tanks to keep up with the demand.

Ways to Lower Temperatures

  • Increase the total airflow and air exchange in your growing room
  • A simple trick: “lights off” during the day — “lights on” at night
  • When you grow with High Pressure Sodium lights (HPS), consider using a cool tube
  • Install an air conditioning unit (AC’s will also lower humidity levels)

Raising Humidity

  • You can temporarily increase humidity levels by misting with a spray bottle
  • Keep the surface of the soil moist, but not wet
  • Hang wet towels/cloths in your grow room or put open water containers in it
  • The most consistent option will be: Invest in a humidifier with a large water reservoir (the need for constant refills makes smaller ones inefficient)

Raising Temperatures

  • Upgrade to a bigger grow light (more watts = more heat)
  • Place a heating mat at the bottom of your tent or grow room
  • Use a simple space heater with a reliable thermostat

Protecting Your Investment

Though it may be exciting to see the fruits of your labor as you begin harvesting, the work is far from done. Correctly processing your crop helps prevent any chances of it becoming damaged or rendered non-smokeable. Proper drying and curing are essential to minimizing the risk of mold contamination and it also greatly improves the taste when smoked.


On top of the need for specific temperature and humidity controls throughout the growing process, it’s worth mentioning that humidity level during the drying process is equally important to the quality of the final product.

Too much humidity in your curing space and you might be left with perfectly grown, but moldy cannabis. Nobody wants that; neither the individual user nor the dispensary buyer. Moldy cannabis is easily the fastest way to ruin your reputation as a reliable grower of high-quality buds.

Again borrowing from Royal Queen Seeds expertise, here’s a how-to on drying your valuable crop:

Wet Trimming versus Dry Trimming

The curing process breaks down chlorophyll remaining in the plant, resulting in a less-harsh taste. Remember: harsh buds are not quality buds. These procedures can enhance the shelf-life of a crop, which is highly important with a product like cannabis that isn’t afforded the addition of preservatives.

Wet Trimming

“Wet trimming” involves trimming as soon as plants are ripe. Remove individual branches and proceed to use sharp scissors or shears to precisely trim excess plant matter. As soon as the plants are cut down, the drying process begins. What’s immediately noticeable is how sticky and wet the buds are. While this is an excellent indicator of potential quality, it’s also a breeding ground for contaminants. Buds left in this state will almost certainly be ruined so it is best to act quickly to begin a controlled drying process.

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Widely regarded as the preferred method, “wet trimming” offers the most control and quality assurance. It also has the added benefit of allowing you to collect the “sugar leaves” which contain lower cannabinoid levels and can be stored separately to be processed into edibles at a later time. Remember that “High THC; low Cannabinoid level oil” states list from before? This is where that product comes from.

Dry Trimming

This technique is usually reserved for growers with a large amount of plant matter and little time to process it. This involves cutting off branches and hanging them whole from drying lines. Once the plants are dry, they are then trimmed and processed. It is more difficult to achieve the neatness of a wet trim as sugar leaves will curl in toward the buds while drying. The agitation of handling a dried bud during trimming can also result in the loss of potent resin crystals.

Drying Room Conditions

Whichever method you choose for trimming your product, suitable conditions for your drying room are important. You’ll want a cool, dark environment with temperatures in the range of 59-71°F and humidity at or near 50% . Yes, even after the growing process is completed, you still need to be concerned with temperature and humidity.

The Drying Process

If you opt to use the wet trimming method, you will need to spread them out across a large surface area. Aeration is important so you’ll want to avoid setting the flowers on a surface like cardboard or newspaper as the buds will dry unevenly. Wire mesh or netted drying racks are more beneficial. This allows airflow to reach all sides and lessen the chance of moist air pockets. Make sure to allow yourself enough space to spread your crop out evenly with a small amount of space between buds for the best results.

You may be tempted to speed up the drying process but it has been shown that slow and steady wins the race. Rushing the process and exposing your crop to high temperatures will almost certainly reduce the quality, and result in diminished flavor and even potentially a “poor and uncomfortable high.” Good luck explaining that to your investors or to a discerning dispensary that has numerous growers vying for their shelf space.

Here again is the equipment you can use to help you attain these levels:

  • Air conditioner
  • Dehumidifier
  • Humidifier
  • Portable heater
  • Thermo hygrometer to measure temperature & humidity levels

Check on your crop frequently, approximately every three to seven days. They will become noticeably drier with each check. You’ll know your buds are properly cured once they can snap easily in your hands without any signs of stringiness. Any bendiness when twisting slightly will tell you there’s water still present and that they’re not ready to move to jars yet.

Find the Best Air Cleaner for Your Grow Room

Pure n Natural Can Help

Pure n Natural Systems specializes in the sale of commercial and industrial humidifiers, dehumidifiers, drying fans, air movers, and portable heating and cooling solutions. We have the equipment you need to make your growing operation a consistent success. And as we’ve already discussed, consistency is king.

That said, we are not experts in growing specifics or in the design of custom-growing spaces.

However, if you say you need to remove 50-gallons of water a day from the air in a warehouse setting, we can help!

Grow Room Humidity Control for Marijuana Plants

One of the perks of indoor growing is cultivators can fine-tune and control their environments. This includes the monitoring temperatures, and humidity control too. By dialing in a grow room’s environment, growers can improve plant health and maximize yields, overall.

Of course, the concept of humidity control is no simple feat to conquer. Especially considering the number of external factors that lend to increased moisture in the air. So, what are some of the best practices for indoor growing humidity control?

Keep reading for everything you need to know on humidity control, including the top tips on how to lower humidity in grow tents.

Why Humidity Control Matters to Plant Growth

First thing’s first, let’s uncover why humidity control matters to the overall growth of your plants. By definition, humidity measures the level of water vapors in the air. To plants, humidity is crucial to evaporation and transpiration. Evaporation and transpiration are two important parts of photosynthesis. That being, the process in which plants convert light energy into chemical energy to grow.

For instance, if humidity levels are low – plants will absorb more nutrients and water. If humidity levels are high, plants can go ‘on guard’ and close their stomata for protection. This will ultimately halt growth, and stunt growth overall since your plant won’t be absorbing food and water optimally.

In addition to reduced and slowed growth, other issues can arise from high or low humidity levels like –

White powdery mildew

White powdery mildew, commonly known as PM is a fungal disease that can sprout from high humidity levels. It begins as small patches of what looks like white powder, and spreads rapidly. So, be sure to check your plants carefully, closely, and frequently if your humidity levels are higher than normal. To avoid powdery mildew, ensure your room has proper airflow and ventilation.

Bud mold or bud rot

Bud mold or rot is most common in late flowering stages and often occurs inside the bud before it appears on the outside. As your plants’ bloom, ensure your room has proper airflow and check the interior of buds for golden brown coloring, which is the first sign of bud rot or mold.

Nutrient deficiencies

The most common signs of nutrient deficiencies in cannabis plants, are yellowing, curling, leaves, or leaves with burnt tips.

That means humidity control is imperative to healthy cannabis plant growth. If you’ve ever grown indoors before, you know just how many factors go into controlling its environment. In fact, factors like temperature, airflow, light types, and even the number of plants can affect the area. So, just how does temperature affect humidity control? We’ll explore that next.

How Temperature & Humidity Connect

Let’s get back to science class, with a brief breakdown of how temperature and humidity connect. Since the relationship will affect how effectively you control humidity levels in your indoor grow space. Relative humidity refers to the measure of water that’s being ‘held’ in the air. In comparison to the maximum amount of water that the air temperature can hold.

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Confused? Perhaps an example will help.

Temperatures around 32℉ hold around 5 ml of water, while 68℉ can hold up to 7.2 ml of water. Hence, why the winter air is drier versus warmer seasons. This idea is often referred to as ‘relative humidity’, which is now being monitored by commercial and home growers alike through Vapour Pressure Deficit , AKA VPD.

That’s because cannabis plants will flourish at varying relative humidities, which ultimately depends on the temperature of the air. To measure both, VPD chart weed measurements are widely available (see below). These charts can give you an idea of how optimally your plant is absorbing and uptaking nutrients and water at varied stages of life.

A benefit of measuring your environment’s VPD is to avoid issues that can arise from too much moisture in the air. Like dew forming on leaves, which can quickly and easily turn into molds. To keep a close eye on humidity levels and temperatures, equip your space with a hygrometer and thermometer that accurately measure both.

You can see from the VPD chart weed ranges will vary for each stage of growth. Next, we’ll dive deep into optimal humidity levels for each stage of growth so you can adjust accordingly.

Humidity Levels for Each Stage of Growth

Naturally, cannabis plants require varying levels of humidity at different stages of growth. This means you’ll be responsible for adjusting your environment as your plants grow from seed to harvest.

Below is a cannabis humidity chart you can reference that reflects optimal levels for each stage.

Stage of Growth Optimal Humidity Levels
Seedling Around 70% Seedlings and clones should be kept in an area with high humidity levels – around 70%. Hence, the use of clone ‘domes’ and wet paper towels to promote moisture during this young stage of plant growth.
Vegetation 40-60% In Veg, you’ll want to decrease humidity levels just slightly. That’s because higher levels of humidity can help your plant thrive, without a healthy strong root system in place. Plant leaves, which develop vigorously during veg have the ability to pull water vapor from the air, therefore growing faster with increased levels of air moisture. An optimal range will be anywhere from 40-60%.
Flowering 40-50% In Flower, another slight decrease will benefit the health of your plants. Since the plant now has a strong root system, it’ll be able to absorb water and nutrients optimally there. In flower, leaves can still absorb water so dropping humidity levels help to avoid molds and mildew that can sprout from too much moisture. The optimal range for ideal humidity control is 40-50%
Late Flowering 40-45% To avoid bud rot forming in the late stages of growth, it’s recommended to lower humidity levels to 40-45%. It’s also believed that dropping humidity levels in late flowering, may increase resin production.

Keep in mind, it’s just as important to consider the exact strain you’re cultivating when it comes to humidity control. Because different strains originated from differing climates, some will benefit from higher levels of humidity versus lower.

To be sure, always acquire your genetics from a trusted and reputable seed bank . As seed breeders should indicate the strain’s preferred climate in its descriptions.

Now that you better understand just why humidity control is important, it’s time to discover just how to control it. Last but not least, is our tips and tricks on how to lower humidity in grow tents, and rooms, and increase it when needed, too.

Top Tips for Controlling Humidity

So now you know just how important humidity is and how it affects your marijuana plants. But how exactly are you supposed to control it? It’s easier than you think.

Start by making a small investment in a thermometer that will also give you the humidity of the room. After all, you can’t fix a problem if you don’t know that it exists. These are little wall units that will tell you the exact environment your plants are in. So you’ll know whether to increase or decrease the humidity levels. To increase, decrease or maintain humidity in general – here are the top tips for humidity control for indoor grow rooms.

Add a Humidifier/Dehumidifier

Up first is the most obvious solution to humidity control. By adding a humidifier and dehumidifier, you can adjust both pieces of equipment to effectively maintain an optimal level of humidity. Most dehumidifier models are now equipped with digital tools and internal hygrometers to maintain the exact level of humidity set. Adding an AC unit can also help decrease levels of humidity.

If you don’t have the funds to invest in a humidifier, there are some DIY options you can implement. Like hanging wet towels or cloths in your grow area. Or, using open water containers. With these methods, you’ll want to be sure to replace the towels or containers frequently, to avoid the growth of mold.

Improve Ventilation

If your grow space is too hot and humid, improving the room’s ventilation is a simple fix. You can do so by increasing the settings on your current exhaust system. Or, you may need to install a more powerful exhaust fan depending on the exact environment.

Keep Soil Surfaces, Moist

When trying to increase humidity, keeping the surfaces of your soil moist is another simple solution. On the flip side, if you’re trying to decrease humidity, avoid overwatering or saturated soil.

Improve Drainage

As we mentioned, standing water can add to humidity levels. So if you’re looking to decrease humidity improving drainage will help. First, reduce the areas in your grow room or space, that collect water. While being sure to remove any water collectors promptly to avoid increased levels of humidity.

Seal and Insulate

Ensuring that your grow room or tent is properly sealed and insulated, will help humidity control altogether. Rooms can be affected by the outside climates, when not properly sealed. So, to dial-in an optimal environment, proper insulation from the exterior’s cold or hot temps will help.


Plants that are extra leafy can add to indoor humidity levels. That means defoliating can help decrease the level of humidity. This step can be crucial to avoid molds or mildews, especially in rooms without proper airflow or ventilation.

Humidity Control for Optimal Growth

From this in-depth guide, now you’re more aware of just how crucial humidity control is for the health of cannabis plants. With a few simple supplies, and a cannabis humidity chart handy – you can easily help maximize growth and yields overall.

Stay tuned for more tips and tricks to optimize your home grow for professional results. Or, begin shopping MSNL’s exclusive collection of clean, trusted genetics. That comes equipped with climate and humidity preferences.

What Is The Best Humidity For Growing Cannabis Plants?

Cannabis is a high-value plant that has come to be primarily cultivated indoors due to its high sensitivity. Each stage of cannabis growth requires a specific kind of tune-up to achieve optimum growth and obtain high-quality yields. Humidity not only plays a roll in the growth of this incredible plant, but also in the proper curing of cannabis, and in proper storage containers and conditions while in storage.

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While many cannabis growers identify sensitivity to the condition of the soil and the nutrients available for the plant, it’s environmental conditions that really matter. One of the environmental factors that affect the growth of cannabis is humidity.

Humidity can be defined as the amount of water vapor in the air. Generally, there are three types of humidity:

  • Specific humidity – The specific humidity of an area is the ratio of the amount of water vapor to dry air.
  • Absolute humidity – This is the actual amount of water vapor present in a given area.
  • Relative humidity – This is the ratio of the actual amount of water vapor present in an area compared to the speculated maximum amount of water vapor that the area can hold.

In cannabis growth, as with any other plant, the relative humidity is used. It is also worth noting that humidity and temperature are interrelated. The amount of water vapor that air in an area can hold increases with increasing temperature.

What Factors Affect Relative Humidity Control For Cannabis?

There are three basic factors that weigh heavily in determining the correct relative humidity when growing cannabis plants. They are region, genetics, and stages of growth. Although there are lessor and differing situations, these three have the greatest impact.


Cold regions have less moist air due to the temperature relation stated above. Growing cannabis in these areas means that the relative humidity must be increased to an optimum level. On the other hand, hot or tropical regions require less control of humidity as most cannabis plants are highly favored by relatively higher humidity.


A good number of cannabis varieties have been developed over the years. Some cannabis species are adapted to warmer and more humid areas while others are adapted to cold and less humid areas.

Growing Stages

From germination to late flowering, the cannabis plants call for changes in the humidity levels. Since each stage comes with distinct metabolic reactions, the relative humidity needs also vary.

Why Is Humidity Important For Cannabis Plants?

To understand how humidity affects the growth of cannabis, let’s first understand the plant’s respiration process.

The sugars produced in the photosynthesis process must be converted to energy used to support plants’ growth. To convert the sugars, mainly glucose to energy and oxygen, later released to the environment, the cannabis plant must acquire carbon dioxide from the environment.

The carbon dioxide enters the plant via the stomata; tiny pores on the leaves. When this process occurs, some water from the plant’s water reservoir is lost to the environment. Relative humidity kicks in here as it dictates how much water the cannabis plant will lose to the environment.

Two scenarios can occur:

When the air is moist (high relative humidity)

The water concentration gradient is significantly reduced due to the higher amount of water vapor present in the air. This means that the cannabis plant will lose less water and hence remain in its best shape.

When the air is dry (low relative humidity)

The high water gradient created between the plant and the surrounding air causes high water loss. The marijuana plant will try to prevent this by closing the stomata, a process that compromises the vital respiration process.

In summary, relative humidity is vital to cannabis plants as it regulates the respiration process and its subsidiary activities such as water loss. For a cannabis plant to benefit fully from relative humidity, it must be maintained at an optimum level. It will also help if the humidity is also kept constant for an extended period.

Should Humidity Be Low Or High For Cannabis Plants?

Cannabis plants are grown in a relative humidity range of 70-40%. This is neither too high nor too low humidity. As discussed above, both extremes of relative humidity are potential causes of cannabis plant stagnation. It is also good to understand that high humidity may support the growth of mold that may affect the growth of your marijuana plants.

What Humidity Should I Have In My Grow Tent?

When you cultivate cannabis in a grow tent, conditions such as humidity and temperatures must be appropriately controlled. While by now we know that different growing stages of the cannabis plant require different levels of humidity, it would help if you kept the humidity in your grow tent relatively high for cannabis seedlings. Several factors affect the amount of humidity in your cannabis grow tent. These include:


If the lights of your grow tent are always on, the temperature of the contained air is raised and hence the relative humidity. Poorly lit grow rooms have cold and dry air (low humidity).


If your grow tent is adequately ventilated, the relative humidity is reduced due to increased air circulation. Ventilation is required for mature cannabis plants that require low relative humidity.

Frequency of Watering

Watering cannabis plants in a grow tent increases the plant’s water intake through the roots. High uptake of water by the plant increases the water loss rate through the leaves, making the room more humid. The induced moist environment, in turn, regulates the respiration process keeping the marijuana plant healthy. Again, watering should be optimized to maintain the grow tent in the recommended level of humidity.

What Humidity Is Good For Cannabis Flowering?

The recommended relative humidity for a flowering cannabis plant is 40-50%. The moisture is reduced to this level mainly to prevent the growth of mold that is very vicious at this stage.

Did you know that mold infestation on one cannabis flower can ruin your entire harvest? Lowering the humidity in this stage is, therefore very crucial and should not be overlooked. The humidity can also be reduced below 40% when the cannabis is close to harvest. This, however, depends on how your cannabis plants respond to changes in humidity.

In conclusion, it can be said that humidity is an essential condition in the growth of cannabis plants. Always start with a high relative humidity of about 70% for your cannabis seedlings and reduce the moisture gradually until the plant is ready for harvest, where the humidity should be around 40%.

Luckily, humidity levels in cannabis growing spaces can be detected easily using manual or automatic hygrometers. If your cannabis plants are in a grow tent, there is also an advantage of automatic reduction of humidity levels. Commercial size dehumidifiers and fans are used for this purpose.